The present invention relates to an air conditioning system for the air conditioning of a space, in particular for the air conditioning of an aeroplane cabin, comprising a heat exchanger for the cooling of a compressed air flow (tapped air flow) originating from an engine or an auxiliary unit and comprising at least one expansion device per air cycle machine (ACM) for the expansion and cooling of compressed air, with the expansion devices being disposed after the heat exchanger and being in connection with a mixing chamber or the space to be air conditioned on the outlet side.
Such air conditioning systems are required, for example, for the air conditioning of an aeroplane cabin in order to heat or to cool the cabin, to set the desired pressure level in the cabin and to provide passengers and crew with fresh air. Such previously known air conditioning systems for aeroplane cabins as a rule include a first heat exchanger and a second heat exchanger which pre-cool tapped air led out of the engines or out of the auxiliary engines, with the cooling performance of the heat exchangers being provided by means of blowers arranged in the stagnation air channel in ground operation and as a result of the stagnation pressure during flight. Furthermore, so-called air cycle machines (ACMs) are generally provided which consist of at least one turbine stage having a blower or—respectively and—a compressor which are fixedly arranged on a shaft. The compressor compresses the compressed air flowing from the first heat exchanger and then guided into the second heat exchanger. A high pressure water extraction circuit generally adjoins this, is disposed in front of the turbine and extracts a substantial part of the moisture from the compressed air before it is led into the turbine. The air dehumidified in this manner is expanded and cooled in the turbine, subsequently slightly heated by means of the high pressure water extraction circuit and subsequently led into a mixing chamber of the aeroplane. Here, the conditioned air is mixed with air of the aeroplane cabin guided in the circuit and subsequently supplied to the aeroplane cabin.
In particular when such air conditioning systems are used in aeroplanes, it is important that the system works largely free of error and that, in the event of an error, a substantial loss of performance or even a total failure can only occur with a very low probability. For these reasons, it has been proposed that the less reliable components such as the air cycle machine are doubly present and the reliable components such as the heat exchangers and the high pressure water extraction circuit are only provided once. Such an air conditioning system is known, for example, from U.S. Pat. No. 5,704,218.
The functioning of an air conditioning system with a doubly made air cycle machine and with a singly made high pressure water extraction circuit and singly provided heat exchangers will be explained with reference to FIG. 3:
Hot compressed air 1 (tapped air) is supplied to the system from the engines or an auxiliary unit at 1.5 to 3.5 bar and 150° C. to 230° C. In ground operation, the tapped air is extracted from an auxiliary engine and supplied to the system at approximately 3 bar. The tapped air flow is initially led through a primary heat exchanger (PHX) and hereby cooled to approximately 100° C. Approximately half of this compressed air in each case is compressed to approximately 4.5 bar in the compressor C1 (2) or in the compression C2 (3) respectively. The temperature of the compressed air flow which has been compressed amounts to approximately 160° C. After the merging 4, the compressed air flow is cooled to approximately 45° C. in the heat exchanger SHX.
The air is now led into the so-called high pressure water extraction circuit. This comprises a condenser CON and a heat exchanger REH disposed before the condenser. The water extractor WE is disposed after the condenser. The compressed and cooled tapped air is cooled by approx. 15° C. in the condenser and the condensed water is separated in the water extractor. The heat exchanger heats the air discharged from the water extractor by approx. 5° C., which is necessary to evaporate the remaining moisture present in the compressed air before the air is guided into the turbines. The heat exchanger furthermore has the task of correspondingly pre-cooling the compressed air before entry into the condenser.
After being discharged from the heat exchanger REH, the air stream is split up and around half in each case is guided into the turbine T1 (5) or T2 (6) respectively. The compressed air expands here to the cabin pressure of approx. 1 bar. The air is furthermore cooled down to approx. −30° C. at the turbine outlet. After the merging 7 of the expanded air streams, the air is led through the cold side of the condenser CON, with it being heated to approx. −15° C.
The air conditioned in this manner is mixed with recirculated cabin air in the mixing chamber 8 not shown in any more detail.
Each of the cooling turbine units ACM1, ACM2 (9, 10) consists of at least one turbine stage with a blower or compressor—in the example in accordance with FIG. 3 of the three wheels compressor 2, 3, turbine 5, 6 and blower 21, 22. These three wheels are fixedly connected to one another by the shaft. In ground operation, the blower 21, 22 and the compressor 2,3 are actively driven by the turbines 5, 6. In flight operation, the compressor takes up almost all the turbine performance. The stagnating air amount can optionally be restricted by controllable flaps 15, 16 at the inlet of the stagnating air channel 11 or at the outlet of the part channels 13. The heat exchangers SHX and PHX are arranged in the stagnating air channel 11. After flowing through the heat exchanger PHX, the stagnating air is split among the two part channels 13 at the point 12. After flowing through the part channels 13, the air flows back to the environment.
The temperature control of the cooled air supplied to the mixing chamber 8 takes place, as a rule, by means of one or two valves TCV 14 and the stagnating air flaps 15, 16.
A typical malfunction in the air conditioning system described in FIG. 3 is the failure of a cooling turbine unit (air cycle machine ACM). It must also be ensured in this case that a specific amount of cooled air and cooling performance are available. Two valves SOV1 and SOV217, 18 are provided for this purpose, by means of which the inlet sides of the turbines 5, 6 can be cut off. Furthermore, two check valves 19, 20 are provided in the region of the inlet of the compressors 2, 3.
If, for example, the cooling turbine unit 9 arranged at the top in FIG. 3 fails, the check valve 19 prevents the air compressed by the compressor 3 being in operation from flowing back via the compressor 2 and thus again being applied to the inlet side of the compressor 3. The check valve 19 prevents such a useless circulation flow. Furthermore, the valve SOV117 is closed so that the air compressed by the compressor 3 does not flow over the stationary turbine 5, but only over the functioning turbine 6 and is expanded here.
Since in the event of such a malfunction the required total air amount is nevertheless required, the functioning cooling turbine unit ACM2 (10) would now have to convey 100% of the air flow, which is, however, not possible since each ACM is only desired for approx. 50% of the required total air amount. It is therefore necessary to bypass the non-functioning cooling turbine unit by a bypass line. The valve TCV 14 is arranged in this bypass line and when it is opened, some of the tapped air 1 is fed directly in front of the condenser CON and then delivered to the mixing chamber 8.
Such a procedure has the disadvantage that the hot air flowing over the valve 14 is mixed with the cooled turbine air at the turbine outlet (after the mixing point 7), whereby the cooling performance of the system is correspondingly reduced. The loss in cooling performance must be compensated by larger heat exchangers (PHX, SHX), which corresponds to increased space requirements and weight. Both parameters are, however, subject to tight limits, in particular in aeroplane design, so that such a solution is unsatisfactory.
Alternatively to this, it would be conceivable to compensate the failure of an ACM in part in that both ACMs are oversized. It would be conceivable, for example, to design each of the ACMs for 70% of the total throughput. The disadvantage of a higher weight or higher dimensions of the ACMs also results in this case.
The above-described bypass of a cooling turbine unit ACM is not only required when this is defective, but also serves to increase the transmission of the air conditioning system. This is necessary since there is a tendency with aeroplane engines for the pressures for the tapped air to become lower and lower. The increase in the transmission is achieved via the valve TCV 14 with the above-described bypass. The disadvantage also exists in this case that hot air is mixed with the cold air from the turbine, which reduces the cooling performance correspondingly. In this case, too, the loss in cooling performance would have to be compensated by larger heat exchangers, which is, as indicated above, not wanted.
Both cases described above, that is the failure of an ACM and the further opening of the TCV to increase the transmission of the air conditioning system, result in the SHX which is mainly responsible for the heat discharge to the outside is only flowed through in part. The whole cooling potential of the SHX can thereby not be used, which has the disadvantage that this has to be compensated by larger heat exchangers PHX, SHX and corresponding increased weight.